Fuel cells are electrochemical devices that convert the chemical energy of reaction directly into electrical energy. The basic physical structure of a single fuel cell includes electrodes (an anode and a cathode) and an electrolyte between and in contact with the electrodes. To produce electrochemical reactions at the electrode, a continuous fuel stream and a continuous oxidant stream are supplied to the anode and cathode, respectively. The fuel cell electrochemically converts a portion of the chemical energy of the fuel in the fuel stream to electricity, while the remaining amount of the chemical energy is released as heat. A stack of individual fuel cells are connected in electrical series to generate a useful voltage, and the byproduct heat may be used for generation of additional electricity by means of a bottoming cycle, such as a steam cycle. To be a competitive alternate source of energy, however, the costs of fuel cell technology must be reduced and performance increased.
The type of electrolyte comprised in a fuel cell is generally used to classify the fuel cell and is also determinative of certain fuel cell operating characteristics, such as operating temperature. Classes of fuel cells under current development are the Polymer Electrolyte Fuel Cell (PEFC), the Alkaline Fuel Cell (AFC), the Phosphoric Acid Fuel Cell (PAFC), the Molten Carbonate Fuel Cell (MCFC), and the Solid Oxide Fuel Cell (SOFC). The operating temperature of a fuel cell strongly effects the fuel cell electrochemical conversion efficiency, the quality of the bottoming cycle, and the cost and endurance of the fuel cell system. The upper and lower limits of the operating temperature of a fuel cell are determined by several factors, including electrolyte stability and vapor loss, the chemical stability of the fuel cell materials, and the thermomechanical characteristics of the fuel cell components (i.e. the ability of fuel cell to withstand thermal stress induced by the temperature gradient across the fuel cell).
High temperature fuel cells include MCFCs and SOFCs, which operate at temperatures of about 650.degree. C. and 1000.degree. C., respectively. High operating temperatures are beneficial to fuel cell performance because of increased reaction rates, higher mass transfer rates, and lower cell resistance due to the higher ionic conductivity of the electrolyte. High temperature fuel cells are also attractive because operating temperatures of greater than 600.degree. C. allow internal reforming of the fuel stream, an endothermic reaction useful for controlling the internal operating temperature of the fuel cell. Additional advantages of high temperature fuel cells are the production of sufficiently high temperature heat for generating steam for use in bottoming cycles, such as gas turbine cycle, and the use of less expensive catalysts than those required in lower temperature fuel cells. Disadvantages of operating fuel cells at high temperatures include limitations on the materials selected for fabrication, problems due to interfacial reactions among adjacent components, and high mechanical stresses resulting from the differential thermal expansion of adjacent materials.
In operation, the flow and utilization of the continuous gaseous fuel stream creates a temperature gradient across the fuel cell stack. As fuel is consumed in the electrochemical reaction at the anode surfaces, the operating temperature of the fuel cell rises due to the heat of reaction, increasingly activating the electrochemical reaction. The fuel cell must be cooled for efficient generation of electricity. In addition, the reaction at the anode reduces the percentage of fuel in the fuel gas as it flows through the fuel cell, progressively reducing the amount of fuel consumed in electrochemical reactions, such that the electrochemical reactions become slightly more inactive. Although the thermodynamic conversion potential decreases as the fuel cell operating temperature increases, higher reaction rates, higher mass transfer rates, and lower cell resistance usually result in a net positive impact on energy conversion efficiency at higher temperatures.
Conventional methods for cooling fuel cell stacks, in order to comply with the temperature limits of the fuel cell materials, incorporate heat transfer elements between fuel cell stacks and/or supply excess oxidant to the fuel cell system. Disadvantages of using external heat exchangers are increased maintenance and capital equipment costs. Similarly, circulating or recirculating oxidant in excess of the stoichiometric requirement for the reaction of oxidant on the cathode electrode decreases system efficiency and increases capital and operating costs.
Conventional approaches to increase the efficiency of a fuel cell system are based on the principal of maximizing fuel utilization (the amount of total fuel supplied that reacts electrochemically) in any single fuel stage. Fuel sharing and splitting configurations, wherein fuel and air streams are networked in series and/or parallel flow arrangements, are designed to improve fuel utilization by maintaining a uniform operating temperature across the fuel cell system, such that all the fuel cell stacks operate at the same temperature and each fuel cell stack operates at its maximum efficiency. Achieving uniform fuel distribution is critical to the successful operation of the fuel cell stack, as damaging hot spots are caused within the fuel cells by uneven or mal-distribution of the fuel gas.
A need continues to exist in the art for a reduction in cost and increase in performance of fuel cell systems.
The present method and apparatus provides a high temperature fuel cell system that substantially reduces the economic and operating inefficiencies of the prior art systems by connecting in series at least two fuel cell stages having different operating temperature ranges and underutilizing the fuel in each stage. According to the present fuel cell system, the pathways for the oxidant and fuel gases, referred to herein as process gases, are provided continuously throughout the system, such that the process gases enter a first, upstream stage, continue to flow through the upstream stage, exit the upstream stage, and flow directly into the next adjacent downstream stage. Each stage is designed to operate within a predetermined temperature range to accommodate the progressively higher temperatures of the process gases as they proceed through the system. A certain percentage of the fuel stream is consumed in each stage, and the system is further designed to underutilize the fuel available in the fuel stream in each stage. Unlike conventional fuel cell systems which maintain uniform operating temperatures from fuel cell stage to fuel cell stage and maximize fuel utilization per fuel cell stage resulting in the need for external cooling systems and fuel networking schemes, Applicants' method and apparatus optimizes the efficiency of the fuel cell system, while substantially eliminating the need for auxiliary cooling and fuel supply means.
Therefore, in view of the above, a basic object of the present invention is to provide a fuel cell system that reduces or eliminates the need for costly, auxiliary cooling equipment by providing a fuel cell system comprised of multiple fuel cell stages connected in series, whereby each fuel cell stage is designed to operate within a predetermined temperature range, and, preferably, whereby each fuel cell stage is designed to operate within a higher temperature range than the operating temperature range of the adjacent upstream fuel cell stage.
Another object of this invention is to provide a fuel cell system that achieves high operating efficiency by allowing significant underutilizing of the fuel within various stages of the multi-stage fuel cell system.
Another object of this invention is to provide a fuel cell system that achieves high operating efficiency by allowing the amount of fuel underutilized per stage to be equal for each stage of the fuel cell system, whereby there is equal fuel consumption in each stage, or by allowing the amount of fuel underutilized per stage to be variable, whereby there is unequal fuel consumption in each stage.
Yet another object of this invention is to provide a fuel cell system that minimizes material and fabrication costs and difficulties associated with high temperature fuel cell designs.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of method and apparatus and combinations particularly pointed out in the appended claims.